U.S. Pat. No. 3,801,183 (Sevelin et al.) discloses a substantially transparent sheet to be used as an overlay on a document such as a credit card, a pass, a license, or a phonograph label to serve as an identifying or authenticating marking. The sheet is retroreflective over its entire surface area and also contains a retroreflecting image such as a pattern or legend which is obscure in that it may be invisible or indistinctly visible to the naked eye under diffuse light and so does not obstruct any underlying visual information. Since the image is either more or less brightly retroreflective than the background areas, it becomes quite visible when viewed under retroreflective light. Because of these attributes, the sheet of the Sevelin patent is widely used to make more difficult the counterfeiting of phonograph records, audio and video cassettes, drivers licenses, vehicle titles, and passports.
A preferred sheet of the Sevelin patent comprises a monolayer of glass microspheres bearing a patterned dielectric mirror which incorporates the aforementioned obscure image. U.S. Pat. No. 4,099,838 (Cook et al.) discloses a sheet similar to that preferred Sevelin sheet, but modified such that reflective areas of the dielectric mirror have differing effective optical thicknesses in the legend and background areas so that the color retroreflected from background areas is different from the color retroreflected from areas of the obscure image. The sophistication of the technology needed to achieve such color contrasts increases the difficulty of counterfeiting.
Other retroreflective sheets containing images, which may be obscure, could be adhesively bonded to documents to make counterfeiting more difficult. For example, see U.S. Pat. Nos. 3,154,872 (Nordgren) and No. 4,082,426 (Brown), but the imaged sheets of those patents are not transparent and so would obstruct information over which they were applied.
Although U.S. Pat. No. 4,200,875 (Galanos) says nothing about counterfeiting, it too concerns a retroreflective sheet which contains an image that may be obscure. The image-bearing sheet is opaque and so would obstruct information if it were adhered to a document for authenticating purposes.